The present invention relates to an electro-hydraulic braking (EHB) system for a motor vehicle.
Electro-hydraulic braking systems for motor vehicles are known which comprise a brake pedal, a braking device connected to at least one vehicle wheel which is capable of being brought into communication with an electronically controlled valve arrangement in order to apply hydraulic fluid under pressure to the braking device, a hydraulic pump, and a hydraulic pressure accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking device via the electronically controlled valve arrangement in order to apply hydraulic fluid under pressure to the braking device in so called xe2x80x9cbrake by wirexe2x80x9d mode in proportion to the driver""s braking demand as sensed at the brake pedal.
In view of the fact that, with an electrically-actuated braking system, the driver""s wishes are acquired by sensors at the brake pedal, and conducted to the electronic control system by means of electrical signals, such systems are described as electro-hydraulic braking (EHB) systems.
In the case of an electro-hydraulic braking system of this form, the braking energy required for braking the vehicle is provided in normal use by the electrically-actuated braking system (xe2x80x9cbrake-by-wirexe2x80x9d mode). In order, however, to brake the vehicle in the event of an unexpected failure of the electrically-actuated braking system, the braking system also features an ancillary hydraulic braking system for the immediate actuation of the brakes in which a direct connection can be established between the brake pedal and the brakes by means of switchover valves and hydraulic lines, (this is referred to as the xe2x80x9cpush-throughxe2x80x9d mode).
Thus, to provide a redundant hydraulic emergency system, a direct connection can be established in the xe2x80x9cpush-throughxe2x80x9d mode between the brake pedal and the brakes by means of switch over valves and hydraulic lines. This conventional system requires a switching device by which, under normal operating conditions, the brake pressure which is produced in the electrical system, and, in the event of a defect in or the failure of the electrical system, the brake pressure produced in the hydraulic ancillary system, is transferred to the brakes.
Of course, the braking system is automatically in the push-through mode if the vehicle is held on the brakes before the system is activated or if the vehicle is free-wheeling under braking before the system is activated. In these and similar circumstances, a problem with known systems is that as soon as the vehicle engine is started and initialization of the electronic braking system begins, the system instantly changes over from xe2x80x9cpush-throughxe2x80x9d braking to xe2x80x9cbrake by wirexe2x80x9d braking, and adopts the braking demand level corresponding to the prevailing push-through brake pedal travel/effort. However, this is much greater than the driver""s actual demand under push-through braking due to the effects of rear-axle braking and the electronic boost ratio and the result is an uncomfortable jolt within the system and vehicle due to the sudden increase in braking. The driver has then to compensate for the braking level which is much higher than expected.
In accordance with the present invention, a push-through condition is recognized during the initialization stage of the EHB, i.e., during power-up of the EHB controller, and the initial EHB demand is set at the prevailing push-through braking or deceleration level and then adapted smoothly to the desired EHB demand.
In some embodiments, after the initialisation stage of the EHB mode, the initial EHB demand is held for a predefined time interval at the prevailing push-through braking or deceleration level, at least for the front brakes.
In some embodiments, it is preferred that the adaption is arranged not before the brake pedal is first released after the initialisation stage of the EHB mode, at least for the front brakes.
Preferably, the adaption follows a substantially straight line characteristic from the initial demand set at the prevailing push-through braking or deceleration level to the desired EHB demand, at least for the front brakes.
Preferably, initial EHB brake pressure for the rear brakes is set at zero and gradually adapted, either by a curved or straight line, to match the EHB brake pressure for the front brakes.
Preferably, the existence of the push-through condition during EHB initialisation is recognized by any one or combination of the conditions that: (a) both front brake pressures are equal to the master cylinder pressure and are greater than zero, (b) pedal travel is greater than zero and within a range of travel expected for push-through at that pressure, and (c) both rear brake pressures are zero.
Advantageously, the initial EHB demand for the front axle brakes is set at the prevailing master cylinder pressure level, the initial EHB demand for the rear axle brakes is set at zero, and the final EHB demand level for both the front and rear axle brakes being set at a predetermined function of prevailing master cylinder pressure.
Preferably, the maximum rear axle demand rise rate is set to be equal to the ratio of the final EHB demand level and a desired adaption period.
The above objects as well as other objects not specifically enumerated are achieved by a method of controlling an electro-hydraulic braking (EHB) system for a vehicle of the type which operates normally in an EHB mode wherein hydraulic pressure is applied to braking devices at wheels of a vehicle in proportion to a driver""s braking demand as sensed electronically at a brake pedal, but which, if a brake-by-wire mode should fail, operates in a push-through mode wherein hydraulic pressure is applied to the braking devices at the vehicle wheels by way of a master cylinder coupled mechanically to a brake pedal, characterized by: arranging for the existence of a push-through condition to be recognized during an initialization stage of the EHB mode; and setting an initial EHB demand at a prevailing push-through braking or deceleration level, and then adapting the initial EHB demand smoothly to a final desired EHB demand.
A system that can be operated according to the above method is also disclosed.